(1) Field
The embodiments discussed herein are directed to a communication method, a user equipment and a radio base station in a radio communication system. The embodiments may be used in communication when the user equipment randomly accesses the radio base station so as to establish a radio link, for example.
(2) Description of Related Art
There is a case in which a Random Access CHannel (RACH) is used as a method for a User Equipment (UE) to establish the radio link with a Base Station (BS), in the radio communication system.
A sequence in which the UE performs a cell search and is allowed to transmit the RACH signal in the Long Term Evolution (LTE) specification being discussed in the 3GGP is depicted in FIG. 21. Meanwhile, the cell search is disclosed in the Section 5.1.7.3 of Non-Patent Document 1 to be described below.
The BS 100 cyclically transmits each signal of a Synchronous CHannel (SCH), a Reference Signal (RS), and a Broadcast CHannel) as a downlink (DL) signal.
Herein, the SCH is the channel used for an initial cell search by the UE, and a primary SCH (P-SCH) and a secondary SCH (S-SCH) are defined such that the UE 400 can hierarchically identify a pattern of the RS to be used to estimate the downlink propagation channel.
The RS is a signal corresponding to a common pilot signal used for an existing W-CDMA system and is a known signal (pattern) between the BS 100 and the UE 400 used for estimating the downlink propagation channel.
A plurality of RS patterns are prepared, and are grouped into a plurality of groups for the hierarchical pattern identification. That is to say, it becomes possible to identify the group of the RS patterns by information of the P-SCH and to identify the RS patterns belonging to the group by the information of the S-SCH.
The BCH is a channel used for transferring information to be notified from the BS 100 to the UE 400, such as information regarding the RACH preamble.
Upon receiving the P-SCH and the S-SCH cyclically transmitted from the BS 100, the UE 400 firstly identifies the group to which the RS patterns transmitted by the BS 100 belong based on the information of the P-SCH, and secondly identifies the RS patterns out of the group identified based on the information of the S-SCH.
Then, the UE 400 estimates the DL propagation channel from the BS 100 using the identified RS pattern, receives the BCH to collect the BS information, and recognizes a transmittable series of the RACH preamble and timing thereof. Thereby, the UE 400 is allowed to transmit the RACH preamble for establishing an initial connection, as needed.
Meanwhile, the sequence as described above is similar to that in an existing W-CDMA system. Also, regarding the transmission of the SCH, a technique referred to as Precoding Vector Switching (PVC) is discussed in the LTE, refer to Non-Patent Document 2 to be described below. This will be described in detail below.    [Non-Patent Document 1] 3GGP TS 36.300 V8.2.0[online], Oct. 5, 2007, 3rd Generation Partnership Project (3GPP), [searched on Dec. 11, 2007], Internet <URL: http://www.3gpp.org/ftp/Specs/html-info/36300.htm>    [Non-Patent Document 2] 3GPP TSG RAN WG1 Meeting #48 bis (R1-071630), NTT DoCoMo, Fujitsu, et al., “Transmit Diversity Scheme for SCH in E-UTRA”, Mar. 26-30, 2007
Since the RACH preamble is used by the UE to perform uplink (UL) transmission to the BS for the first time for establishing the initial connection, the BS does not receive the signal to estimate the UL propagation channel such as the DL RS before receiving the RACH preamble. Therefore, there is a case in which the reception quality of the RACH preamble in the BS is degraded.